Relativistic corrections to polarized-tritium β decay

Forthcoming experiments such as Project 8 and Ptolemy aim at investigating with high precision the end point of the tritium β-decay spectrum sensitive to the neutrino mass. In light of this, using the standard parametrization in terms of nuclear polar form factors, we analyze the complete relativistic expression for the spectrum of the β electron emitted by a tritium nucleus. Given the small parameters in the problem, we systematically discuss the approximations that can be made, and present the first two corrections to the standard lowest-order formula. We particularly discuss the case of an initially polarized target, and the consequences on the spectrum as a function of the neutrino mass. We show that, while it induces an angular anisotropy that can be measured by future experiments, such anisotropy cannot be used as an additional handle to constrain the neutrino mass.